pgsql: Add support for IN as alternative to FROM in PL/PgSQL's FETCH

Log Message:
-----------
Add support for IN as alternative to FROM in PL/PgSQL's FETCH statement,
for consistency with the backend's FETCH command. Patch from Pavel
Stehule, reviewed by Neil Conway.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        plpgsql.sgml (r1.107 -> r1.108)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.107&r2=1.108)
    pgsql/src/pl/plpgsql/src:
        gram.y (r1.100 -> r1.101)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y.diff?r1=1.100&r2=1.101)
    pgsql/src/test/regress/expected:
        plpgsql.out (r1.52 -> r1.53)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out.diff?r1=1.52&r2=1.53)
    pgsql/src/test/regress/sql:
        plpgsql.sql (r1.45 -> r1.46)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plpgsql.sql.diff?r1=1.45&r2=1.46)